The present invention relates to a seismic processing method.
The general principle of seismic prospection consists in using a seismic source to cause a disturbance in the subsoil and to use sensors to record seismic traces as generated by said disturbance so as to extract information therefrom concerning the geology of the subsoil, and in particular to detect the presence of hydrocarbons.
Two types of wave propagating through the subsoil are conventionally distinguished, namely: compression waves (so-called type P waves) which vibrate in the propagation direction; and shear waves (so-called type S waves) which vibrate perpendicularly to the propagation direction.
As a result, seismic traces picked up on the surface can correspond to a plurality of modes of propagation from the seismic disturbance. In particular, the disturbance can be a compression wave while the surface sensors record the compression components of waves reflected on various interfaces in the subsoil (P mode propagation) or the shear components thereof (converted wavesxe2x80x94PS mode propagation). The disturbance could equally well be a shear wave, in which case the surface sensors record shear waves (S mode propagation).
Conventionally, the geological materials studied by a reflection seismic survey are characterized by their densities xcfx81 and by the propagation speeds Vp and Vs of the compression and shear waves. In particular, it is possible to express the reflectivity of an interface as a function of three parameters. The equations making this possible are known as Zoeppritz equations and they make use of the angle of incidence of the wave on the interface in question.
Seismic inversion consists in calculating these three parameters xcfx81, Vp, and Vs on the basis of seismic traces recorded in a surface seismic survey.
An object of the invention is to propose a seismic processing method that makes it possible quickly and robustly to determine the various parameters xcfx81, Vp, and Vs.
To this end, the invention takes advantage of the fact that it is possible to make two independent estimates of the ratio Vp/Vs.
It can be estimated from the vertical paths of the S waves and of the P waves.
In this respect, reference can be made to the following article:
M. D. McCormack, et al.: xe2x80x9cA case study of stratigraphic interpretation using shear and compressional seismic dataxe2x80x9dxe2x80x94Geophysics, May 1984, USA, Vol. 49, No. 5, pp. 509-520, XP002143569.
The ratio could also be estimated from the reflectivities of two (or more) wave modes reflected (or converted) at the interface in question.
The invention seeks to take advantage of these two possible ways of making an estimate. Specifically, the invention provides a method of processing seismic traces corresponding to two modes of propagation in the subsoil, the method being characterized by the following steps:
determining at least one pair of correlated events from the seismic traces corresponding to the two modes respectively;
determining the ratio of the differences between the vertical travel times Ts and Tp in the S domain and in the P domain, corresponding to said correlated pairs of events; and
determining the values of Ts and Tp over the time interval defined by the two pairs of correlated events so that throughout said time interval, the ratio rt=Ts/Tp is equal to the ratio Vp/Vs of the speeds in the P domain and in the S domain, and so that the mean value of said ratio r over said time interval corresponds to the ratio of the differences between the vertical travel times corresponding to the pairs of correlated events.
Such a method is advantageously associated with the various following characteristics taken singly or in any technically feasible combination:
to optimize the ratio rt=Ts/Tp, over a plurality of subintervals defined by sampling times ti distributed over the time interval in question, an estimate of xcex4Vp/Vpxe2x88x92xcex4Vs/Vs is determined as a function of reflectivity parameters corresponding to the two modes of propagation respectively, where xcex4Vp and xcex4Vs respectively represent the variation in speed in the P domain and in speed in the S domain over such a subinterval;
for the various sampling times ti of the time interval in question, the values of Ts and Tp are determined so that for each subinterval as determined by the sampling times ti, the relative differential variation xcex4rt/rt is equal to the estimate of xcex4Vp/Vpxe2x88x92xcex4Vs/Vs as determined for said subinterval;
for the various sampling times ti of the time interval in question, the values of Ts and Tp are determined so that for each subinterval as determined by said sampling times ti, the relative differential variation xcex4r/r, convoluted with the seismic wavelet, is equal to the estimate of xcex4Vp/Vpxe2x88x92xcex4Vs/Vs as determined for said subinterval;
the seismic traces correspond to P and S propagation modes;
to estimate xcex4Vp/Vpxe2x88x92xcex4Vs/Vs, the following is calculated:
(xcex4r/r)a=2[Rp(0)xe2x88x92Rs(0)]
where Rp(0) and Rs(0) are the P mode and S mode reflectivities at normal incidence;
the seismic traces correspond to P propagation mode and to PS propagation mode; and
in order to estimate xcex4Vp/Vpxe2x88x92xcex4Vs/Vs, the following is calculated:
(xcex4r/r)a=xc2xd[(4+r)/Rppxe2x88x92rGpp+(2+r)Gps]
where Rpp, Gpp, and Gps are P mode and PS mode reflectivity parameters.
The invention also provides a seismic prospecting method in which a disturbance is emitted into the subsoil and sensors are used to pick up seismic traces corresponding to two different propagation modes, and in which a seismic treatment method of the above-defined type is implemented to provide information about the geology of the subsoil.